Cartogirl

Posted 18 November 2008 - 12:08 PM

I am working on a project for my company and this is one of the first times they have really stepped into the GIS world so to speak. More often then not CAD is used for data presentationetc.

I have been asked to explain how arc deals with differnt cell sizes. For example I have slope data at 50x50m and Bathy data at 50x50m of the same area. The cell sizes are the same and the area is the same, but that doesn't mean the points are all in the identical locations. They could be a few metres in any direction.

What my boss wants to understand is how Arc deals with this situation when a particuar query is made. If he is looking for a area where bathy,slope and sediment classification are in line with one another, how do we know that nothing has been left out because of cell differences. Do the cell sizes "read" from the centre node?

Sorry if this is somewhat unclear, that is why I am asking. I am able to use the tools to do what i want but find it very difficult to verbalize the results.

dsl

Posted 01 December 2008 - 02:42 AM

dsl

Master Contributor

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Based on this Post and your Previous (?) post about importing the data, this is what I can say.

A raster is created, essentially, using an origin, cell size, and area. The origin is a starting point, and the width and height of an area are used to calculate how many cells are inside the raster grid. The center of the cells is determined based off the origin point. While the GIS displays the cell value for the whole cell, if you convert the raster to points then it is the center of the cell that actually has the value. If you get the x,y coordinates of the cell, it is the center of the cell that those coordinates refer to. If you right-click on the raster layer, and choose properties all this information is provided for you. Or if you export the raster as an ascii txt file, this information is in the header bit of the text file. So presuming that your slope and your bathymetry rasters have the same origin, height, width, and cell size, then they should line up exactly. Values of course will be different because you are looking at different information.

Since it sounds like the data originally came from points, you must have converted it to raster at some point. Are the points supposed to be the same exact points? Was there an interpolation performed? What do you mean by "query"?